The location method based on electromagnetic signals has been described, in a very general way independent of the subject of application, e.g. in patent publications U.S. Pat. Nos. 5,747,996, 4,346,384 and DE3326476. In one embodiment, the device comprises a set of signal sources, a set of receivers and one or more signal generators which are used to generate a set of transmitter signals known as concerns their tense to be transmitted by signal sources. In addition, the aforementioned patent publications disclose an analysis method for processing the output signals of the receivers and for using them in calculating the position of an object in relation to another object. Common to the devices described in the publications is that the signal transmitters have been attached to the object in a manner geometrically rather restricted.
In addition, in publications U.S. Pat. Nos. 5,747,996 and 4,346,384 one requires that the signal sources are orthogonal between themselves. Thanks to the orthogonality, there is no correlation between the signals transmitted by the signal sources, i.e. the signals do not have an effect on each other that would disturb the determining of location. In addition, in publication U.S. Pat. No. 5,747,996 one requires that the receivers are coils placed on the same level. The geometric requirements are used to facilitate and speed up the signal analysis and to eliminate the possible sources of error having effect on the positioning result.
In the co-ordinate system of an object, the location method based on known signal sources is used e.g. in magnetoencephalography (MEG), in which one measures the weak magnetic fields generated by the neural activity of a human being or other organism that are dependant on time and place. Based on the measured magnetic field values, one tries to locate the source areas that generated the observed field. In magnetoencephalography, the head of a testee is as close as possible to the set of detectors i.e. receivers consisting of extremely sensitive supraconducting detectors, the geometry of which is known. The position of the head in relation to the measuring device is determined using, as known signal sources, coils attached to the surface of the head the magnetic field generated by whom may be approximated by the field of a magnetic dipole.
As receivers, the measuring detectors of the measuring device are used that are also used for the receiving and measuring of the actual brain signals to be measured. The basic principles of the method have been described e.g. in publications SQUID' 85: Super-conducting Quantum Interference Devices and their Applications, 1985, pages 939-944 and Proceedings of the 7th International Conference on Biomagnetism, 1989, pages 693-696.
The actual MEG measurements are usually implemented as repetition measurements, in which a response generated by the brain, followed e.g. after a certain stimulus is measured several times successively, and a mean value of the measurement results time-locked in relation to the stimulus is calculated. When using the mean value of the measurement results the effect of noise may be attenuated by a factor which is vice versa proportional to the square root of the number of repetitions. One problem with the repetition measurements is their long duration, because of which the head of a testee may move during the measurement. From this automatically follows that the position of the source of the response generated by the brain changes in relation to the measuring device in the middle of measurement, thereby causing errors to the final analysis.
Traditionally, the position of the head has been determined solely in the beginning of the measurement so that each head positioning coil has been activated and the magnetic field generated has been measured one by one, in which case the location method has been rather slow. After the location, the testee has been asked not to move his head until the end of the repetition measurement.
The errors resulting from the movement of head during the measurement may be avoided by a continuous measurement of position. In that case, one has to be able to use the measuring device simultaneously also for the measuring of other transmitter signals than the one to be generated in the positioning. One way to eliminate the effect of the transmitter signals on the useful signal to be measured, i.e. on the response signal generated by the brain is to set the frequencies of the transmitter signals far away from the frequency band to be examined and to filter measurement data appropriately in the frequency plane. This kind of solution is presented in publication Biomag2000, 12th International Conference on Biomagnetism, Book of Abstracts, p. 188, Peters, H. et al. Another solution is the filtering of the transmitter signals from the output signals of the receivers by subtracting the shares corresponding to the transmitter signals from the measured signals, in which case one has to known the strengths and wave shapes of the transmitter signals to be measured.
When trying to determine the position of an object constantly or repeatedly at short intervals the signal transmitters have to be activated simultaneously and one has to be able to tell the difference between the simultaneous components generated by different transmitters and the measurement signals. The method should distinguish the frequency components as efficiently and accurately as possible using a data collection time as short as possible. In a generally used distinction method the frequencies and the data collection time are adjusted so that the signal components are orthogonal between themselves at the time interval being examined. If the phase of the transmitter signal is known, then the amplitude of each signal component is achieved directly by calculating the projection of the signal vector consisting of the measurement results for the basis vector corresponding to the signal component being examined that consists of the computational values of the basis function known as concerns its frequency. Applications based on the orthogonality of the basis vectors have been described e.g. in publications “The use of an MEG device as a 3D digitizer and a motion correction system”, de Munck et al Proceedings of the 12th International Conference on Biomagnetism, Helsinki, Finland. In this description, the effect of the non-orthogonality has been taken into account on a principal level. In the positioning method described, the orthogonalisation of the transmitter signals, however, substantially reduces the amount of computation associated with the positioning, so in an implementation in practice, the transmitter signals are orthogonalised.
The requirement of orthogonality sets requirements to the frequencies to be used as well as to the data collection time, and in addition, the supposition of orthogonality for non-orthogonal signals causes great errors in the computed amplitude coefficients and thereby also in the positioning. In the signal analysis described above one tries to use signals collected from a time interval as short as possible in order that the positioning would be as real-time as possible and the movement of the objects would be as slight as possible during the data collection of the positioning measurement. Dependable positioning measurements have been made in magnetoencephalography using a data collection time of solely 100 ms.
Even at a time interval like this objects may move, which makes the positioning result worse. Due to the possibly large movement of the objects, the strengths of the signals measured by the receivers may vary from the lowest limit of a signal to being observed rapidly up to the upper limit of the dynamic area of the receivers. The variation may be significant especially in small distances, since the strength of a measured signal is vice versa proportional to the third power of the distance of objects. In addition to this, the same transmitters may be used in different measurements for objects for very different sizes and located in different distances in relation to the transmitters. The repeated succeeding of the measurements to be made in different situations requires that the strengths of the transmitter signals measured by the receivers constantly remain within certain limits. The problem has been solved by using an adjustment algorithm which controls the power of the transmitters in such a way that the amplitudes of the signals measured by all the receivers remain above a certain lowest limit and below a certain upper limit. The return switching, or feedback, of the transmitter signals has been described e.g. in patent publication U.S. Pat. No. 5,747,996.